1. Field of the Invention
The present invention relates to a power supply circuit for display unit and a display unit outputting a plurality of power supply voltages.
2. Description of Related Art
Semiconductor integrated circuit is used as a circuit driving a liquid crystal display unit. In the liquid crystal display unit, power may repeatedly be turned on and off when a screen saver or the like is activated. Therefore, in the circuit driving the liquid crystal display unit, a large amount of power is consumed by frequently repeating ON/OFF. Therefore, reducing of power consumption is required in the circuit driving the liquid crystal display unit.
On the other hand, Japanese Unexamined Patent Application Publication No. 2000-37036 (Oda) discloses a technique reducing power consumption in the circuit. Oda discloses the technique disconnecting a capacitor and a power supply stabilization circuit and holding the charges stored in the capacitor when voltage is not output from the power supply stabilization circuit so as to reduce power consumption.
However, it is needed in the circuit driving the liquid crystal display unit to output a plurality of voltages to drive the liquid crystal display unit. Therefore, latch-up may occur if ON/OFF is repeated while charges are held in the capacitor. Specifically, decreasing level of the voltage is different between capacitors because the capacitance in the capacitor connected to the power supply circuit varies and leak current flowing in the power supply circuit also varies. Because voltage decreases due to this difference in the capacitance, when the power supply circuit stops working, magnitude correlation of voltage due to the residue charge in the capacitor may be reversed. When the circuit driving the liquid crystal display unit is activated again, latch-up tends to be occurred.
In the conventional technique, latch-up is prevented by grounding both terminals of the capacitor to discharge charges in the capacitor every time the power of the liquid crystal display unit driving circuit is turned OFF (see Japanese Unexamined Patent Application Publication No. 2004-151514 (Kobayashi), and No. 2002-6811 (Ito), for example). FIG. 5 shows a block diagram of the display unit discharging charges of the capacitor according to the related technique.
As shown in FIG. 5, a display unit driving circuit 90 of the display unit includes a power supply circuit 92 connected to a logic circuit 91, a source driver 93, a gate driver 94, a common driver 95, and an oscillator (OSC) 96. The logic circuit 91 is connected to a central processing unit (CPU) 97 and receives control signal from the CPU 97 for operating. The source driver 93, the gate driver 94, and the common driver 95 are connected to a panel for display 98. Further, capacitors C91, C92, and C93 for power supply stabilization are formed externally to the display unit driving circuit 90 and connected to the display unit driving circuit 90. The charges stored in the capacitors C91 to C93 are discharged when the display unit does not work. When the power of the power supply circuit 92 are turned on in order to drive the display unit driving circuit 90, it is possible to prevent the occurrence of the latch-up between drivers connected to each of the capacitors C91 to C93 and the capacitors C91 to C93 because there are substantially no charges in the capacitors.
However, in the conventional display unit, charges stored in the capacitors C91 to C93 are discharged when voltage is not output from the power supply circuit 92 even when ON/OFF of the power supply circuit is repeated in such a short time that natural discharge of the capacitor does not cause any problem. Therefore, it is needed to store charges again in the capacitors C91 to C93 when voltage is output from the power supply circuit 92 again. This causes increase in power consumption in the related technique.